New air-pollution limits for diesel engines have caused some manufacturers to adopt selective catalytic reduction (SCR) technology for reducing nitrogen oxides (NOx) in engine exhaust. The SCR process introduces or injects an NOx reductant (e.g., a NOx reductant-water solution) into the hot exhaust gas, which chemically reduces NOx into non-pollutant compounds. When introduced, the NOx reductant-water solution undergoes a decomposition process that releases ammonia into the exhaust stream to facilitate NOx reduction in the SCR process.
One type of prior art diesel engine exhaust system that employs an SCR process is illustrated in FIG. 1. As best shown in FIG. 1, a diesel engine 4 produces NOx as a component of an exhaust stream. The exhaust stream is directed from an exhaust port (not shown) of the engine 4 to a doser or injector section 8 through an exhaust pipe 10. The doser or injector section 8 includes a NOx reductant doser or injector 12 that is configured to inject a solution of NOx reductant (or other nitrogen oxides reductant) into the exhaust stream present in the doser or injector section 8.
After the NOx reductant doser or injector 12 injects NOx reductant into the exhaust stream, the exhaust travels through an exhaust pipe 14 to a catalytic section 16, where the NOx reductant and a catalyst within the catalytic section 16 react to reduce the nitrogen oxides into non-pollutant species. The reduced nitrogen oxides and remaining exhaust components are then directed out of the vehicle through an exhaust outlet pipe 18.
However, during typical operation of an engine, the conditions for the SCR decomposition process are not optimal (e.g., exhaust is too hot or cold) and either too much, or too little, NOx reductant solution is introduced into the exhaust. When introduced or injected NOx reductant solution fails to undergo the SCR decomposition process (e.g., too much NOx reductant solution for the exhaust temperature), NOx reductant crystals will accumulate within the exhaust system, both on the interior surface of the exhaust pipe and at the port that introduces the solution into the exhaust. Build up of NOx reductant crystals in the exhaust system detrimentally affects the performance of the exhaust system, and also is indicative of inefficiency in the SCR process: NOx reductant crystals represent both wasted NOx reductant solution and reduced SCR efficiency.
During injection of NOx reductant into the diesel exhaust stream as part of the SCR process there is a significant amount of the total NOx reductant injected that contacts the walls of the exhaust pipe and becomes a liquid wall film. While this process occurs, the NOx reductant that is wetting the pipe walls does not reach the catalyst for its intended use and the intended quantity of reductant is not available in the catalyst.
Prior attempts at avoiding such “wall wetting” NOx reductant build up have been only moderately successful. The primary method for avoiding NOx reductant build up is the use of a limiting table whereby a NOx reductant injector limits NOx reductant injection to a predetermined amount based on average engine conditions as defined by a lookup-table. The limiting table is primarily focused on preventing NOx reductant build up and, thus, the resulting SCR performance is compromised because the amount of NOx reductant reaching the catalyst is typically less than is required for optimal SCR performance.
Therefore, a system and method is desired for optimizing the injection of NOx reductant solution into the exhaust of an operating diesel engine such that the SCR catalyst is provided sufficient NOx reductant while NOx reductant build up prior to the catalyst is minimized.